Endothelial cells appear to play a major role in the control of hemopoiesis in vitro. Studies have demonstrated production and release of hemopoietic growth factors for pluripotential and committed erythroid, myeloid and megakaryocytic progenitors. Molecular probes have also shown induction of message for some of these factors, in cultured human umbilical cord derived endothelium. However, the mechanisms of control of production of these factors remain largely unknown. This proposal outlines studies which will examine the effects of specific cytokines on the physical and biological characteristics of vascular endothelium,including the induction of production of hemopoietic growth factors, the relationship between the cytokines and specific receptors at the cell membrane level and the role of culture conditions for full expression of the activity of cultured endothelium. Furthermore, these studies will attempt to address the biological role of human microvascular (bone marrow derived) endothelial cells in culture, which may help us understand the mechanisms of 'homing' of transplanted hemopoietic stem cells. Several methods will allow us to isolate and propagate these cells in vitro which will permit us to examine and characterize the interactions of these cells with human hemopoietic stem cells. These studies will investigate and compare the biological characteristics (such as production of hemopoietins, production of extracellular matrix proteins, arachidonic acid metabolism, cell surface antigenic composition) of bone marrow derived endothelial cells with umbilical cord derived cells. Moreover, we will attempt to isolate and characterize the factors or components which regulate the interactions between marrow cells and endothelial cells. The proposed research will help us understand the signals which regulate hemopoietic proliferation and differentiation and which are mediated by cells or cell-derived substances.